Sailboat airfoil sail and mast assembly

ABSTRACT

A sailboat mast assembly including a curved mast and a sail draped thereon to extend to both sides of the mast and leewardly thereof. Two booms are pivotally mounted at the base of the mast and are attached to the lower edges of each portion of the sail on each side of the mast, and the booms can be pivoted upwardly into compact position with the mast and they can be swung horizontally away from the mast to spread the sails apart for downwind sailing. Struts extend between the mast and the booms for tensioning the sail. Pulley and rope combinations are applied to the mast and booms for aligning these parts with respect to each other in the plane of the curvature of the mast and in the horizontal swinging of the booms. A bearing structure is mounted on the boat for the upright support of the mast and for quick and easy installation of the mast and sail assembly onto the boat.

United States Patent [72] inventor Clarence E. Keuney Primary Examiner-Trygve M. Blix Attorney-Arthur .I Hansmann 119 Stuart Road, Racine, Wis. 53406 1 1,051

[21 Appl. No. [22] Filed Feb. 13, 1970 [45] Patented Aug. 10, 1971 ABSTRACT: A sailboat mast assembly including a curved 5 SAILBOAT my-01 SAIL AND MAST ASSEMBLY mast and a sail draped thereon to extend to both sides of the 10 cu 1; Dn'ing Figs. mast and leewardly thereof. Two booms are pivotally mounted [52] U5.

at the base of the mast and are attached to the lower edges of each portion of the sail on each side of the mast, and the booms can be pivoted upwardly into compact position with the mast and they can be swung horizontally away from the mast to spread the sails apart for downwind sailing. Struts extend between the mast and the booms for tensioning the sail. Pulley and rope combinations are applied to the mast and booms for aligning these parts with respect to each other in the plane of the curvature of the mast and in the horizontal swinging of the booms. A bearing structure is mounted on the boat for the upright support of the mast and for quick and easy installation of the mast and sail assembly onto the boat.

114/39, 114/102 v...B63b 5/00, B63h 9/00 114/39, 102, 103

References Cited UNITED STATES PATENTS 7/1951 Wel1s-Coates................ 114/102 1/1967 114/103 [51] Int. 1-

[50] Field of PATENTEDAUBIOISYI 3,598,075

am 1 [1F 2 I N 1 5 N TOR. CLARENCE E KENNE) SAILBOAT AIRFOIL SAIL AND MAST ASSEMBLY SAILBOAT AIRFOIL SAIL AND MAST ASSEMBLY This invention relates to a sailboat airfoil sail and mast assembly.

BACKGROUND OF THE INVENTION faces, and combinations of the two. None have been generally accepted, often because of excessive weight or cost.

My invention includes means which will result in an assembly approaching an airplane wing in efficiency and still be light enough (especially at the top) to be successful in propelling a sailboat.

' The elements of this invention are:

5. The strength of the large diameter mast, and the reduced force between it and the boat, permit the use of a'single support for the mast. Therefore, the mast of a large boat can be erected by a small winch carried onboard. Thus, independence of shore-based cranes facilitates launching a boat with this assembly at any of the numerous ramps now used primarily by motorboats. The corresponding ease of disassembly permits this boat to be taken home for safe, inexpensive storage.

The means of this invention and other commonly known devices which can be used to obtain these objectives are further described herein.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side elevational view of a preferred embodiment of this invention, with parts thereof broken away.

l. A single assembly of any suitable sail material constructed so that it passes around the front of a cylindrical mast to form two smooth, continuously curved effective surfaces of an airfoil, regardless of the side of the assembly held toward the wind;

2. The cylindrical mast is light, thin-walled and tapers from a small top to a much-larger diameter than usual at the bottom, both for strength and to become the leading edge of the airfoil. The diameter of the mast is such that the resulting thickness of airfoil is substantially proportional to the chord at all horizontal planes on the assembly. The mast is preferably curved backward to form a curved tip, in contrast to the sharp triangular peak of conventional sails. This curve is also helpful in holding all sections of the sail more nearly at the same or most effective angle with respect to the wind;

3. The bottom of each of the two effective sail surfaces is fastened to a separate boom. The two booms are held loosely together whenever an airfoil section is required, as when sailing into or across the wind. When sailing with the wind, the two booms can be separated and swung one to each side of the boat to double the effective area of the sail. This avoids the major disadvantage of boats having only one sail and usually known as a catboat;"

4. Each boom is forced downwardly by a separate strut of an adjustable length and extending from each boom to a common bracket attached to the mast at a point above the attachment of the booms to the mast. The struts permit universal sideways swinging so that the booms can swing apart. This force exerted by the struts provides the tension in the sail, particularly in tension members near the trailing edge of the sail, to shape the sail and to hold all sections toward the wind. There is a space between the effective sail surfaces, and it encloses the struts to avoid significant interference to smooth airflow. The sturdy mast and resulting airfoil thus permits use of the struts. An important advantage of this invention is the increased safety and ease of control possible when only horizontal force is required to position the sail. The sail can be held by hand or by conventional fasteners which would release quickly should the wind, and therefore the force on the sail, suddenly increase to an amount approaching that which would overturn the boat. Means of preventing flow of air between the boat and the sail, and other means of value in shaping the sail, become practical when the need for vertical external force on the sail is eliminated. The force transmitted by the mast to its support is also reduced when the sail-shaping force is contained within the mast and boom assembly;

FIG. 2 is an enlarged sectional view taken along the line 22 of FIG. I, and with arrows showing the sail angles and force vectors.

FIG. 3 is an enlarged rear elevational view of a fragment of FIG. I.

FIG. 4 is an enlarged sectional view taken along the line 4-4 of FIG. 1.

FIG. 5 is a sectional view taken along the line 55 of FIG. 1.

FIG. 6 is a side elevational view, similar to FIG. I, and showing a fragment of that shown in FIG. 1, but with the sail I removed.

FIG. 7 is a top plan view of FIG. 6.

FIG. 8 is a front elevational view of FIG. 6.

FIG. 9 is an enlarged sectional view taken along the line 9-9 of FIG. 1.

FIG. 10 is an enlarged sectional view taken along the line 10-10 of fig. 1.

FIG. 11 is a sectional view taken along the line 11-11 of FIG. 10.

FIG. 12 is a bottom plan view of FIG. 11.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 shows a mast, sail, and support assembly according to this invention. The mast 15, must be both strong and light and preferably curved, with the tapered shape shown. A practical method of manufacturing such a mast is to roll a strip of light metal into a curved spiral with the edges overlapping enough so that they can be joined by welding or by an adhesive on the spiraled lines shown. The thickness of metal forming the mast I5 is chosen to produce a mast with low deflection at the maximum expected load. The width of the strip is chosen to obtain the taper required by the design.

' The sail is made of any suitable material and by conventional methods. It can be in one piece, or in two,joined as by a zipper. A carefully planned fullness to obtain sail curvature is contemplated in this sail, as it is in a conventional single thickness sail. The thickness of sail in front of the mast is exaggerated in the'drawings, for clarity. The sail is raised to the top of the mast 15 by a rope 16 passing over a pulley l7 and attached to a reinforced peak of the sail by a separable fasten IS. A drum l9 fastened to an accessible part of the mast I5, and used to hoist the sail and lock it at any desired height, is shown. The sail itself consists of the near side sail surface or portion 20 and the far side surface or portion 21. The bottom of each sail surface is fastened to a separate boom 22 and 23, respectively. Part of the near side sail surface is removed to show two struts, shown generally at 24 and 25, connecting between a common bracket 26, on the mast 15, and each of the booms 22 and 23. The struts are separately adjustable in length and are the primary means of tensioning the sail surfaces to shape the sail.

Two tension members 27 extending in a straight line from the reinforced sail peak to each sail foot at the trailing edge are shown in FIGS. 1, 2, and 3. They are preferably flat, like a tape, or are multiple thicknesses of sail material, and are built into the inside of the sail surfaces.

FIG. 2 shows the shape of the sail and its position in relation to the boat and the wind for one representative condition of sailing. The forward underside of the sail, as seen in FIG. 21, is always toward the wind, at an angle approximately as shown. When the wind is from the opposite side of the boat, curvature and position of the sail with respect to the boat are reversed.

Characteristics of airfoils and forces developed by them are commonly defined by perpendicular distances to each sail surface and from a chord line and by vectors perpendicular to or parallel to the chord line. The chord length 28 of FIG. 2, and the maximum spacing between the two sail portions, as shown at 29, are approximately proportional at all sections which may be taken across the height of the sail. At any setting of the sail, the angle between the center line 30 of the boat and the chord line 28 can have a small controlled increase, with the height of the sail, to accommodate the change in the relative wind,-as shown by vector 31, which is'the vectorial sum of the value of the actual wind, as indicated by vector 32, and the boat speed, as indicated by vector 33. The increase in actual wind velocity with the height of the sail, which increase requires this change in angle, is not constant so means are provided to adjust the rate of change in angle of the sail with height of the sail. The angle between the relative wind and the chord line 28 is indicated at 34 and is the angle of attack. In an airfoil, the lift, as shown by vector 35, perpendicular to the chord 28, and the drag by vector 36 and net force by vector 37, resulting from airflow across the airfoil, change rapidly with the angle of attack. Ability of this system to hold all sections of the sail at substantially the same angle of attack contributes to the efficiency. The forces on the sail sections are proportional to the chord length of the various sections across the sail, thus one section represents the entire sail. Components of the net sail force parallel to the centerline 30 of the boat, and shown by vector 38, add to obtain the total force available to propel the boat. Forces perpendicular to the centerline of the boat add to give the sideways force 39. This force must be balanced by the force which the keel or equivalent part of the boat develops as it moves through the water. The sideways forces must be multiplied by their distances above the center of buoyancy of the boat to obtain the overturning moment which must be resisted by the stability of the boat. This factor requires that a sail be lower and more tapered in its height, than an equivalent airplane wing. A

- sail designed according to this invention is less restricted in height than a conventional sail. The top can contribute useful force with relatively low drag which is of added advantage aloft where the wind is stronger than at the surface. The top of a conventional sail is often at zero angle of attack and contributes only drag.

The tapered cylindrical section of the mast is affixed to a straight section 40 to complete the mast. The lowest part of this cylinder is fitted to rotate inside the cylindrical part of the support shown generally as 41, and is prevented from listing by any conventional fastener at 42. Pivot plates 43 at either side of the boat rotate on pins 44 held by brackets345 which are part of the boat. The pivot plates are connected to the cylinder of the support by the truss members shown. The mast is erected on the boat by first raising and rotating the socket support 41 to bring its longitudinal axis horizontal. The mast is inserted into the support 41 and locked in place. A hoist 46 is used to raise the mast to the vertical position shown in FIG. 1, where it can be locked by a clamp 47. The hoist 46 can be removed and stored until needed to lower the mast. The brackets 45 and clamp 47 are the only mounting attachments of the mast to the boat and must be strong enough to withstand all forces llpon the mast. It should be obvious that the support for a small mast of a small boat can be a sleeve secured through'a hole in the deck of the boat. The small mast could then be liftedby hand and placed in its support.

The means provided to attach the booms 22 and 23 to the mast satisfy operation of the sail as an airfoil, and with booms separated, and it must also permit the booms to be folded close to the mast 15 for storage. Reference is made to FIGS. 6, 7, and 8 in describing the mechanisms designed to accomplish these objectives. The boom 22 is fastened to a bracket 48 which pivots upon a shaft 49 carried by arms of a ring 50. In the same way the other boom 23 is bolted to bracket 51 pivoting on shaft 52 carried by ring 53. The rings 50 and 53 have a free bearing fit around the cylindrical part of the mast 40 and are restrained vertically between rings 54 and 55 which are clamped to the mast. When the sail is being used as an airfoil, and the booms are held together, a set screw 56, on a lug which is part of the upper clamp ring 54, is incontact with one side of ring 50. In the same manner, the ring 53 contacts a setscrew 57, in a lug on the lower ring 55. As the rings 54 and 55 are clamped to the mast, the mast is positioned with its curved upper section aligned with the booms, within a margin provided by adjustment of the setscrews 56 and 57. The shafts 49 and 52 are located so that when the boom struts 24 and 25 are disconnected, the booms can pivot upwardly to lie together close to the mast.

When the booms 22 and 23 are separated and swung apart to sail with the wind, the mast must be held approximately centered between the booms to obtain equal curvature of the sail surfaces 20 and 21 now extended to opposite sides of the boat. This is accomplished by a flexible cable 58 passing from a spring-loaded anchor 59 on the ring 50, and the cable extends over the' ring 50, over a pulley 60 on clamp 54, downwardly and around a pulley 61 on clamp 55, and over a part of the circumference of ring 53 to an anchor at 62 in the ring 53. A similar cable 63 extends from a similar anchor or ring 50, over pulleys 64 and 65, and to the ring 53 where it is anchored in any conventional manner. By having the four pulleys on the two clamps 50 and 53, the 'four pulleys are in effect attached to the mast and, in combination with the cables, rotate the mast to a position centered between the separated booms, upon rotating the booms on the mast end 40, and by virtue, of the resultant pulling of the cables 58 and 63.

FIGS. 9, 10, 11, and 12 show details of a design of strut providing easy adjustment of length, lightweight and a separable connection to be used in assembly of struts 24 and 25. The struts 24 and 25 are each an assembly of several parts. They are similar except for overall length so strut 24 is described in detail and represents both struts.

FIGS. 10, 11, and 12 show left-hand threaded rod end 70 secured to the top of strut 24. Rod 70 fits through a matching tapped hole in a ball 71 which can be of light plastic bearing material. The ball is held with a sliding fit between bracket 72 and cover 73 by bolts 74 and spacers 75. A key 76, fastened to the ball 71, slides loosely in the bracket 72 to prevent rotation of the ball when the strut is turned to adjust its length. The brackets 72 of both struts 24 and 25 are fastened to bracket 26 by bolts 78. FIG. 9 shows the lower end of strut 24 with a right-hand threaded rod end 79 shown tight in the tube 24 and with a key 80 to prevent rotation, and a detent 81 to hold the strut and rod related. The rod 79 fits through a threaded hole in ball 82 which is identical to 71 except for the hand of the threads. A bracket 83 has a vertical leg shaped to fit inside the boom 22 and is held thereto by a screw 84. A bracket 83 of strut 25 is opposite hand to fit boom 23. A flexible shaft and handwheel 86 and 87, connected on each strut 24 and 25, extend to an accessible location below the booms 22 and 23. They are rotated clockwise to lengthen the struts, and are able to tilt sideways when the booms are swung apart.

The struts 24 and 25 retain forces required to shape the sail within the mast and sail assembly. They are designed to be as short and light as requirements of strength permit. The attachment points are located so that tension in the sail will decrease and the sail become slack as the booms 22 and 23 are swung apart.

The attachment of the trailing edge of the sail to the booms 22 and 23, called the outhaul, is conventional and is described referring to FIGS. 1 and 3. The bottoms of both sail surfaces 20 and 21 are fastened and reinforced around a common foot rope 90. This assembly passes through grooves in the booms as shown in FIG. 4. The diameter is such that it cannot pull out of the groove. The ends of the sail and rope 90 are fastened into additional reinforcing plates 91 and 92. I lates 93 and 94, fastened to theends of the booms, are shaped to hold the trailing edges of the sail together. Holes in the plates align with holes in the sail assembly to receive self-locking retaining pins 95. The spacing of the holes is designed to permit vernier adjustment of sail position. The lower edges of plates 93 and 94 are bent outward to retain the clamp 96 when it is snapped over them to hold the two booms together.

Safe operation of a boat in strong winds requires a means of reducing sail area. This is provided by one or more rows of reefing" chords 97 in FIG. 1 attached to the inside of the sail surfaces 20 and 21 and aligning holes 98 in each boom. They are applied by first separating the booms slightly, lowering the sail, gathering it between the booms until the chords on each surface can be passed through holes in the opposite boom. The booms are then brought together again and the chords tied together below the booms. The sail is then hoisted snug. It will have a harmless bulge if the mast is part straight and part curved, as shown in FIG. 1.

The line provided to position the boom of a sailboat is known as the sheet." The sheet of a conventional boat is fastened to a sliding traveler and passes through other positioning and force-multiplying pulleys to exert a downward sailshaping force as well as to position the sail with respect to the wind. The unique design of this sail system confining the sailshaping forces within the mast and boom structure, requires the sheet 99 to exert only a horizontal force which is proportional to the force of the wind to overturn the boat. For this reason the sheet can be held safely by any conventional fastener which will release it quickly when the force exceeds a preset safe value. It is also easier to hold the sheet by hand. A second sheet 100 of FIG. 1 which is normally stowed between the booms 22 and 23 is used to control the position of the second boom when they are separated for sailing with the wind. This sheet is also required to exert only horizontal force.

The lesser sail-positioning force required permits use of a fillet sheet 101 in place of, or in addition to, the normal sheet 99. This would not be used for leisurely sailing but would be held in reserve to obtain maximum speed. On a boat without a cabin, the fillet sheet would extend along the full length of the booms and be held either by hand or by automatic release fasteners at the side of the boat. It would be slotted to drape around the sailor. A boat designed for use with this sail system could take advantage of the more predictable position of the booms to include a cabin roof parallel to the under side of the booms for as much of their length as practical. Flexible strips 102 of FIGS. 1 and 4 are fastened to the booms to slide across the roof preventing airflow between the booms. The relative length of sheets 101 and 102 will vary to suit the design of the boat that the sail is mounted upon.

Optimum curvature of the sail surfaces is most important to the efficiency of the sail. Flexible strips usually of wood, called battens," are used to help shape conventional sails and are also preferred to be on this sail whenthe trailing edge extends beyond the tension members 27 of FIGS. 1, 2, and 3, to hold the overhanging part of the sail in alignment with the rest of the sail. The battens 103 are retained in pockets of sail material 104 as shown in FIG. 5. The battens are spaced as required on opposite surfaces of the sail so that they do not interfere with each other.

The curvature of a sail depends largely on the fullness built into it, the stiffness of the material and the vertical tension imposed on it. Independent adjustment of the struts provides the different tensions in a surface that may be either the windward or the lee side of the sail. Items 105 and 106 of FIG. 2 are optional strips with matching grooves fastened respectively to the center of the sail and to the front ofthe mast to prevent shifting of the sail and assure independent adjustment of sail tension.

Positive pressure to the windward, and negative pressure to the lee of the sail are in the right direction and location along the surfaces to hold looseness of the sail in the curve required. It is possible to admit air from the positive pressure area of the sail to the space between the surfaces to both decrease the windward and increase the lee side curvatures, often with a net improvement in shape. If this means is provided to shape the sail, leakage of air between the trailing edges and between the booms must be reduced. To accomplish this, flexible strips 107 and 108 of FIG. 2 are built into the tension members 27 with attachment along the batten pockets 104 to hold them both together and resist rotation to open the sealing edges. Similar strips 109 and 110 of FIG. 4, fastened'to the undersides of the booms, extend from the' rnast 15 to where the booms meet at their trailing portions. A flap is shown covering the slot required for the strut adjusting shaft.

A means of admitting air to the interior space of the sail which is unique to this design is to lift part of the fillet seal strip 102 as shown by 111 of FIG. 4 using chords 112. Crosswise sealing plates 116 are provided at each end of the section of fillet lifted and the intergap seal 109 or 110 is omitted or opened in this area. If the boat has no cabin, the same principal is served by shifting the attachment of the fillet sheet 101 from the windward to the lee boom across only the area of highest pressure against the sail. The interboom seals 109 and 110 are opened in this section and remain closed along their remaining length. A more conventional means of admitting air to the intersail space is to provide a number of slots 113 of FIG. 2 and FIG. 1 in the sail surfaces, with flexible flaps 114 which will open automatically to admit air or close to retain it, as required.

It will therefore be understood that the mast 15 is tapered along its length to a smaller cross section at the upper end, and the mast is rotatably mounted in the support 41. With this arrangement, the mast is ofa cantilever mounting and therefore all of the attachments and supporting structure can be incorporated with the mast, as described. The curvature of the lead ing edge of the mast, as designated 115 in FIG. 1, is therefore of a curvature which will provide the same angle of attack 34 at all horizontal sections through the sail, such as the section 2-2. In this manner, the sail is of optimum efficiency.

It will also be understood that the pulleys 60 and 61 are of a smaller size than the pulleys 64 and 65 so that the respective ropes 63 and 58 can pass each other, as seen in FIG. 8. The cable and pulley system function as such that, for instance, upon swinging boom 22 away from boom 23, will loosen the extent of the rope 58 to its connection point 62 on the mounting arms 53 for the boom 23. Therefore, the boom 23 can also swing at an identical angle relative to the swing of the boom 22, but in the direction away from the boom 22, as desired. Upon swinging the boom 22 away from the boom 23, in the example given, tension will be placed in the cable or rope 63, and this will cause the pull on the cable 63 to the point where it connects to the support 53 for the boom 23, and therefore to cause the boom 23 to swing away in an equal angle relative to the boom 22, as mentioned. It should therefore be understood that the booms 22 and 23 are mounted on the mast and arranged therewith so that the booms are oriented relative to the plane of the mast 15, which plane is the one on which the mast is curved.

What I claim is:

l. A sailboat assembly comprising an upright mast, a sail supported by said mast and having two portions extending horizontally from said mast with each of said portions having a rear edge and a lower edge relative to said upright mast and the forward direction of the sailboat, two booms swingably mounted on said mast and extending transversely therefrom for swinging about the longitudinal axis of said mast, each of said lower edges of said sail being attached to a respective one of said booms, pivot means on said booms for uprightly pivoting said booms from the transversely extending position and to a position extending along the length of said mast for folding with the latter, and securing means connected between said mast and each of said booms for forcing said booms downwardly to tension said sail.

2. The subject matter of claim 1, including self-adjusting connectors interconnecting said securing means and said mast for accommodating the various swung positions'of said booms about the longitudinal axis of said mast.

3. The subject matter of claim 2, wherein said securing means are struts capable of transmitting compression forces between said, mast and said booms, and including length adjusting means on said struts for accommodating the various distances between the connection points of said struts on said mast and the connection points of said struts on said booms, in

various swung positions of said booms about said mast, for creating tension in said sail.

4. The subject matter of claim 1, wherein said mast is curved along a plane extending along the longitudinal axis of said mast, and means swingably mounting said booms on said mast for swingably separating said booms to equal distances on opposite sides of said plane.

The subject matter of claim 4, including self-adjusting connectors interconnecting said securing means and said mast for accommodating the various swung positions of said booms about the longitudinal axis of said mast.

6. The subject matter of claim 5, wherein said securing means'are struts capable of transmitting compression forces between said mast and said booms, and including length adjusting means on said struts for accommodating the various distances between the connection points of said struts on said mast and the connection points of said struts on said booms, in

various swung positions of said booms about said mast, for creating tension in said sail.

7. The subject matter of claim 4, wherein said swingably mounting means includes a set of pulleys and ropes, with said ropes interconnected between said booms and said mast and arranged for achieving said equal distances.

8. The subject matter of claim 1, including a socket mounting member for said mast and rotatably receiving the lower end of said mast, and said mast being curved along its length in a curvature presenting at least approximately the same angle of attack of said sail to the wind at all horizontal planes on said sail at all heights of said mast.

9. The subject matter of claim 8, wherein said sail is draped over and in contact with the convex side of said mast, and means for holding said sail to said mast so that said sail portions are'of equal lengths in their extents rearwardly of said mast from said convex side.

10. The subject matter of claim 9, including air inlet openings in both said sail portions for admittance of air between said sail portions and from a location outside said sail portions, and flaps on said sail portions for air-sealing said inlet openings from escape of air from between said sail portions. 

1. A sailboat assembly comprising an upright mast, a sail supported by said mast and having two portions extending horizontally from said mast with each of said portions having a rear edge and a lower edge relative to said upright mast and the forward direction of the sailboat, two booms swingably mounted on said mast and extending transversely therefrom for swinging about the longitudinal axis of said mast, each of said lower edges of said sail being attached to a respective one of said booms, pivot means on said booms for uprightly pivoting said booms from the transversely extending position and to a position extending along the length of said mast for folding with the latter, and securing means connected between said mast and each of said booms for forcing said booms downwardly to tension said sail.
 2. The subject matter of claim 1, including self-adjusting connectors interconnecting said securing means and said mast for accommodating the various swung positions of said booms about the longitudinal axis of said mast.
 3. The subject matter of claim 2, wherein said securing means are struts capable of transmitting compression forces between said mast and said booms, and including length adjusting means on said struts for accommodating the various distances between the connection points of said struts on said mast and the connection points of said struts on said booms, in various swung positions of said booms about said mast, for creating tension in said sail.
 4. The subject matter of claim 1, wherein said mast is curved along a plane extending along the longitudinal axis of said mast, and means swingably mounting said booms on said mast for swingably separating said booms to equal distances on opposite sides of said plane.
 5. The subject matter of claim 4, including self-adjusting connectors interconnecting said securing means and said mast for accommodating the various swung positions of said booms about the longitudinal axis of said mast.
 6. The subject matter of claim 5, wherein said securing means are struts capable of transmitting compression forces between said mast and said booms, and including length adjusting means on said struts for accommodating the various distances between the connection points of said struts on said mast aNd the connection points of said struts on said booms, in various swung positions of said booms about said mast, for creating tension in said sail.
 7. The subject matter of claim 4, wherein said swingably mounting means includes a set of pulleys and ropes, with said ropes interconnected between said booms and said mast and arranged for achieving said equal distances.
 8. The subject matter of claim 1, including a socket mounting member for said mast and rotatably receiving the lower end of said mast, and said mast being curved along its length in a curvature presenting at least approximately the same angle of attack of said sail to the wind at all horizontal planes on said sail at all heights of said mast.
 9. The subject matter of claim 8, wherein said sail is draped over and in contact with the convex side of said mast, and means for holding said sail to said mast so that said sail portions are of equal lengths in their extents rearwardly of said mast from said convex side.
 10. The subject matter of claim 9, including air inlet openings in both said sail portions for admittance of air between said sail portions and from a location outside said sail portions, and flaps on said sail portions for air-sealing said inlet openings from escape of air from between said sail portions. 